The present invention relates generally to an electrical filter and more particularly to a coupling construction of distribution constant type resonators employing a dielectric block for coupling thereof with external circuits in a distribution constant type filter working at a frequency range, for example, at 900 MHz or thereabout as in an application thereof to a radio communication equipment and the like.
Conventionally, for electrical filters to be applied to a frequency range in the order of several hundred MHz, the art has proposed filters which employ LC resonance circuits and coaxial resonators, etc. but most of these filters are unstable or complicated in structure, or their characteristics are not fully satisfactory, and require troublesome procedures for adjustments, without a possibility of reduction in cost.
Accordingly, there has also been conventionally proposed and put into practical application, an electrical filter which employs dielectric coaxial TEM resonators as the filter working at the frequency range of several hundred MHz.
As shown in an equivalent circuit diagram of a two-stage filter of FIG. 1, the known distribution constant type filter referred to above has a circuit construction including input and output terminals Is and Os, respectively, coupled, through input and output coupling electrostatic capacities Ci and Co, to 1/4 wavelength resonance circuits Ri and Ro represented as concentrated constant circuits, thus constituting an electrical filter in which the 1/4 wavelength resonance circuits Ri and Ro are coupled to each other through inductive coupling, while an external circuit and the 1/4 wavelength resonance circuits are also coupled to each other through electrostatic capacitive coupling.
In one example of the specific construction as shown in FIGS. 2 and 3, the prior art distribution constant type filter generally includes a cubic box-like block B made, for example, of ceramic dielectric material of titanium oxide group, through-openings or cavities O1 and O2 formed in the dielectric material block B side by side, with a predetermined space therebetween, electrically conductive layers or inner conductors Eo1 and Eo2, respectively, formed on the inner peripheral faces of the through-openings O1 and O2, and another electrically conductive layer or outer conductor Es provided at least on four side faces of said dielectric material block B. The distribution constant type filter further includes another electrically conductive layer Eb provided on the bottom face of the block B for shortcircuiting one end of each of the inner conductors Eo1 and Eo2 and the outer conductor Es so as to produce 1/4 wavelength resonance circuits thereby, an input coupling capacitor Ci connected to the other end of the inner conductor Eo1 and formed by providing confronting electrodes Ed1 and Ed2 on a cylindrical dielectric member d1. More specifically, to the other end of the inner conductor Eo1, a fixing member n1 made of an electrically conductive member such as a metallic cylindrical member or electrically conductive paste, is electrically and mechanically connected for securing, with the confronting electrode Ed2 being electrically and mechanically connected to the fixing member n1 for being fixed thereat. Meanwhile, there is also provided an output coupling capacitor Co connected to the other end of the inner conductor Eo2 and formed by providing confronting electrodes Ed3 and Ed4 on a cylindrical dielectric member d2. More specifically, to the other end of the inner conductor Eo2, another fixing member n2 made of electrically conductive member, for example, a metallic cylindrical member or electrically conductive paste in a similar manner as in the fixing member n1, is electrically and mechanically connected for securing, with the confronting electrode Ed4 being electrically and mechanically connected to the fixing member n2 for securing thereat. Thus, the resonance frequency is determined by electrical length of the inner conductor Eo1 or Eo2 shortened by the dielectric constant of the dielectric member B. The electrical length may be of 1/4 wavelength or 1/2 wavelength, and in the case of 1/2 wavelength, the bottom conductive layer Eb is not required. It is to be noted that in the drawings, the thickness of the electrode layers and the electrodes, etc. are exaggerated with respect to the actual arrangement for better understanding. In the known arrangement as described so far, two resonance units are constituted, and there is further formed in the dielectric material block B, a cavity V having a cross section, for example, of a rectangular configuration, and the degree of inductive coupling between the two resonance units depends on the dimensions of said cavity V. The inner peripheral surface of the cavity V is not provided with any electrode layer. The cavity V need not necessarily extend through the dielectric material block B.
In FIGS. 4 and 5, there is shown another example of the specific construction of the prior art distribution type constant filter, in which the structure for the electrostatic coupling as described above with reference to FIGS. 2 and 3 is further simplified.
More specifically, in the filter of FIGS. 4 and 5, the input coupling capacitor Ci with the fixing member n1 and the output coupling capacitor Co with the fixing member n2 described as employed in the arrangement of FIGS. 2 and 3 are replaced by respective dielectric units U (FIG. 6) fitted under pressure into the through-openings 01 and 02 formed with the inner conductors Eo1 and Eo2 on the inner peripheral faces thereof as described earlier with reference to FIGS. 2 and 3.
Each of the dielectric units U is provided with a columnar or cylindrical portion U1 having, for example, a circular cross section and formed by applying a dielectric material of plastics or ceramics of titanium oxide group and the like, onto part of a conductive wire U2 having a diameter, for example, of 0.5 mm so that said conductive wire U2 axially extends therethrough, and has a taper portion U3 formed at its forward end for facilitation of insertion of said unit U into the through-openings O1 and O2 of the dielectric material block B, and also, a flange portion U4, for example, of a circular shape formed at its rear end so as to be brought into contact with a peripheral edge of each of the openings O1 and O2 of the block B where the outer conductor Es is not formed. As shown in FIG. 5, the dielectric units U are fitted, taper portions first, into the openings O1 and O2 of the block B formed with the inner conductors Eo1 and Eo2 until the flange portions U4 of the dielectric units U come into contact with the dielectric material block B.
By the known construction of FIGS. 4 through 6 as described above, the conductive wires U2 of the dielectric units U and the inner conductors Eo1 and Eo2 formed on the inner peripheral faces of the through-openings O1 and O2 of the dielectric material block B are electrostatically coupled to each other through the portions of the dielectric material of said dielectric units U, and thus, the input coupling capacitor Ci and output coupling capacitor Co described as employed in the conventional arrangement of FIGS. 2 and 3 may be dispensed with, and accordingly, troublesome procedures required for mounting such capacitors Ci and Co, etc. can be eliminated.
In the prior art arrangements of the electrostatic coupling system as described so far, there are cases where a TE11 mode resonance is produced as a spurious response as shown in FIG. 7. Although cut-off frequencies of TE11 mode are determined by a width "a" and a length "b" in FIG. 7, and particularly, when the arrangement is of a multi-stage construction, the length "b" in FIG. 7 is increased for lowering the resonance frequency of TE11 mode so as to approach the resonance frequency of TEM mode which is the mode employed.
Moreover, when the capacitors are employed for the electrostatic coupling between the resonance unit and the external circuit as in the known arrangement of FIGS. 2 and 3, not only are troublesome procedures required for mounting the capacitors to the dielectric material block B, with a consequent lowering of mass-productivity, but the overall size of the dielectric filter is undesirably increased.
Meanwhile, in the prior art arrangement in which the dielectric units U i.e. external circuit connecting pins are inserted into the through-openings O1 and O2 of the dielectric material block B, it is difficult to improve the structural accuracy, and if any air gap is produced between said through-openings and the dielectric units U, the coupling capacity becomes unstable resulting in the scattering of the characteristics at an initial stage. Furthermore, when the arrangement is subjected to temperature variations, the air gap is altered due to differences in coefficients of expansion, and the coupling capacity becomes unstable with time.
Similarly, in U.S. Pat. No. 3,505,618, there has also been conventionally disclosed a microwave filter which includes a dielectric material block coated with a conductive film on its outer surface to constitute a housing. The block is provided with holes and conductive members may be formed by depositing conductive film on the walls of the holes or they may be formed by a combination of the conductive film and rods which fit in the holes and contact with the conductive film. The filter characteristic may be made adjustable by threading the rods so as to be adjustably screwed in the holes. The prior art microwave filter as described above, however, also has disadvantages as described earlier with reference to the other known arrangements.